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C++ coding conventions and style guide

We try to maintain a consistent coding style because it makes reading and modifying the code easier. Our coding conventions aren't necessarily better than others (though we try to follow ones that make sense and change those that don't), but the main utility of this is consistency.

We're much stricter these days than we were, say, 10 years ago. The reason for this is that a codebase with many contributors and large size is much harder to maintain than a small one. So new code must pass tougher tests than some of the existing code. (Of course, the existing code should ideally be cleaned up more, too, but few people are motivated to work on such tasks if there are other things to do that seem more urgent.) Once code is in, it usually isn't changed for years unless we discover a bug. So we really try to put a lot of effort on making things as right as possible on the first try.

The following is not meant to be a complete description of our coding conventions. When in doubt, follow the example of the existing code.

See also the information on how and where to put ../Whitespace.

Language support

The translator runs on Python >= 3.6. The search code uses C++20 and may use all language features supported by GCC 10, clang 12, MSVC 19.34, and AppleClang 13.

General recommendations

We generally follow the recommendations in the book C++ Coding Standards: 101 Rules, Guidelines, and Best Practices by Herb Sutter and Andrei Alexandrescu. In the tracker or elsewhere, a mark of the form [SA x] is a reference to a rule in that book. For example, [SA 9] refers to Sutter and Alexandrescu's rule 9: "Don’t pessimize prematurely".

Comments

   1 // Write complete sentences ending with periods.
   2 
   3 // Use imperative if possible: "Return the factorial." is better than "Returns the factorial.".
   4 
   5 // Leave a space between the slashes and the comment.
   6 
   7 // TODO: Use this format for todo items.
   8 
   9 /*
  10   Use this style for comments spanning 
  11   multiple lines and strive to write 
  12   self-explanatory code that doesn't need 
  13   comments.
  14 */
  15 
  16 
  17 /* For shorter multi-line comments (2-3 lines)
  18    this style is also fine. */

We generally prefer comments above the code (not next to it). If you really want to put a short comment next to the code, leave one space before and after the // slashes.

Subdirectories, namespaces and cmake plugins

Subdirectories:

• We don't use nested subdirectories (at least for now).
• There are two types of subdirectories:

  • component subdirectories correspond to a single component (e.g. one subdirectory for the PDB code, one for the landmarks code, one for the merge-and-shrink code)

  • grouping subdirectories group together many components that are related and too small to deserve their individual subdirectories (e.g. one for all open list variants and one for all heuristics that don't need their own subdirectory)

• Subdirectories follow the same naming conventions as filenames, methods and variables.
• Subdirectory names should be kept short.

Namespaces:

• We don't use nested namespaces (at least for now).
• Every component (= CMake plug-in) should correspond to a namespace.
  • For components that have their own directory, the namespace name is derived from the directory name.
  • For components that are too small for their own directory, the namespace name is derived from the main file name (or equivalently, the main class name).
• Namespaces follow the same camel-case naming convention as classes.

• We don't use using namespace for our own namespaces.

CMake plugins:

• There's a 1:1 correspondence between namespaces and CMake plugins.

Examples:

• Component directory merge_and_shrink contains the code the merge-and-shrink component in namespace MergeAndShrink.

• The file merge_and_shrink/transition_system.h would be expected to contain the class MergeAndShrink::TransitionSystem.

• Grouping directory open_lists contains the code for various open list components, for example an alternation open list.

• The file open_lists/alternation_open_list.h would be expected to contain the class AlternationOpenList::AlternationOpenList.

Open questions:

• We might introduce a namespace for the core code, i.e., the code that is necessary to build the planner (does not belong to any CMake plugin). Right now it is in the global namespace.

• We might eventually consider settling for shorter class names and/or filenames in cases where the directory name or namespace already provide the necessary context. For example, we might say MergeAndShrink::Heuristic instead of MergeAndShrink::Heuristic; we might say AlternationOpenList::OpenList or Alternation::OpenList instead of AlternationOpenList. (If you want to argue for this, this would need further discussion.)

Header file guards

Macro names for header file guards follow this algorithm:

• Take the filename, including subdirectory name if in a subdirectory.
• Convert to uppercase.

• Replace all "." and "/" with "_".

Example: learning/state_space_sample.h becomes LEARNING_STATE_SPACE_SAMPLE_H.

Guard blocks should look like this:

   1 #ifndef LEARNING_STATE_SPACE_SAMPLE_H
   2 #define LEARNING_STATE_SPACE_SAMPLE_H
   3 // ...
   4 #endif
   5 

That's all. In particular, don't add comments to the preprocessor directives and don't add further underscores.

Includes

Order includes in the following way: header corresponding to .cc file, headers from the same directory, headers from the src/search directory, headers from other directories, standard library includes, and third-party library includes. If using the using namespace std declaration, put it after all other includes. Order each group alphabetically and separate the groups by empty lines. Add an empty line between the last group and the remaining code. Here is a contrived example:

   1 #include "pattern_generation_edelkamp.h"
   2 
   3 #include "zero_one_pdbs_heuristic.h"
   4 
   5 #include "../abstract_task.h"
   6 #include "../causal_graph.h"
   7 
   8 #include "../algorithms/ordered_set.h"
   9 #include "../utils/timer.h"
  10 
  11 #include <algorithm>
  12 #include <cassert>
  13 #include <vector>
  14 
  15 #include <tree.hh>
  16 
  17 using namespace std;
  18 
  19 PatternGenerationEdelkamp::PatternGenerationEdelkamp(const Options &opts)
  20 ...

Constructors, destructors and assignment operators

• Add default destructor only for base classes (i.e., if other classes derive from them). The destructor should of course be virtual.

• Explicitly remove copy constructor, i.e., declare it as = delete, for most types, especially those created by plug-ins and used polymorphically. Generally, we want to permit copy constructions only for cases where we've explicitly decided that it's useful. Many of our objects are heavy-weight and should not be copied.

Function signatures

• Use const methods whenever appropriate.

• Pass strings by const reference.

• When overriding a virtual method, mention virtual again in the declaration and mark it as override (i.e., virtual int foo() override; rather than int foo();).

• Mark function declarations in headers as extern (global functions).

• Mark function declarations in .cc files static (local functions).

const attributes

We generally don't make attributes of classes const. See the attached discussion for the rationale behind this convention. (This is a convention that may be changed if there is sufficient support, but while it is the way it is, we should all follow the same style.) Exceptions include:

• static constants like static const int UNKNOWN = -1; and enumeration values (all cases where we would currently use ALL_CAPS)

• pointers and references to const (not really an exception because we do not mark the pointers as const, i.e., we would write const Frobnicator *frobnicator;, but not const Frobnicator *frobnicator const;.

• attributes referring to plug-in parameters passed in through the option parser (This may seem a somewhat arbitrary convention, and perhaps it is, but for good or bad it does describe the one exception to the general rules that currently occurs commonly in the code.)

The same rules apply to function parameters and local variables.

push_back vs. emplace_back

• For a vector<T>, use push_back with arguments of type T or T&& to copy or move an already constructed object into the container, and use emplace_back in other cases, where the object can be directly constructed inside the container.

  • Rationale: https://stackoverflow.com/questions/26860749/efficiency-of-c11-push-back-with-stdmove-versus-emplace-back-for-already and https://stackoverflow.com/questions/10890653/why-would-i-ever-use-push-back-instead-of-emplace-back

Examples:

   1 Foo x = ...; // construct a Foo
   2 
   3 vector<Foo> foos;
   4 foos.push_back(move(x));    // move the constructed Foo into the container
   5 foos.emplace_back(44, 22);  // Foo has a constructor that takes two ints. Use emplace_back to directly construct the object inside the container.
   6 

Anti-idioms

• Don't write NULL or 0 for null pointers. Use nullptr.

• Don't write (ptr != nullptr). Write (ptr).

• Don't write (ptr == nullptr). Write (!ptr).

• Don't write (seq.size() == 0). Write (seq.empty()).

• Don't write (seq.size() != 0). Write (!seq.empty()).

• Don't append underscores to constructor variables. Use the same name as the member variable (preferrable) or a different name.

Passing and storing tasks

• By default, pass const TaskProxy &.

• Pass const shared_ptr<AbstractTask> & only in the following situations:

  • the callee should participate in the ownership of the task
  • the callee creates a delegating task based on the given task (even if it's only a temporary)

Conceptually, it's less clear that this is desirable, but with our current design you cannot create a delegating task without (co-) owning the task.

• If the callee only needs access to a certain aspect of the task, it is preferable to make this explicit and only pass e.g. const OperatorsProxy & or const VariablesProxy &.

• If the task is needed after an object's construction, store a TaskProxy as a member variable.

• Avoid storing large collections of proxies that carry redundant information. For example, a vector of 10000 FactProxy instances (of the same task) contains 10000 copies of the same abstract task pointer.

• Avoid using proxies in performance-critical code.

Exceptions

Derive custom exception classes from utils::Exception.

• This rule does not apply to internal exceptions never seen by the user, such as HillClimbingTimeout in the PDB code.

• The print method should write to cerr. See the existing examples of utils::Exception subclasses.

• utils::Exception is intentionally not derived from std::exception. See the attached file for some discussion.

Miscellaneous

• Prefer template<typename T> over template<class T>.